Fabrication of water- and oil/grease-resistant paper coating for packaging application

Paper materials are highly desirable for a wide range of applications in packaging and non-packaging sectors due to their advantages of low cost, light-weight, excellent mechanical properties, biodegradability, and renewability. However, paper is a porous material with polar groups that cause it to absorb water and oil. Lamination and coating are commonly used methods to improve the water- and oil-resistance of paper. After use, the separation of laminates and coatings from the paper fibers/pulp presents a significant challenge. In addition, persistent and environmentally harmful fluorinated chemicals are also used to render paper water- and oil-resistant. Thus, there is an urgent need to address the above challenges by developing closed-loop approaches for water- and grease/oil-resistant paper. This research aims to develop facile, economical, and closed-loop approached for fabricating fluorine- and plastic-free water- and grease/oil- resistant paper coating.To achieve PFAS- and plastic-free water-repellent coating for paper, paper substrates were first coated with melamine as a primer, an FDA approved material for food contact applications, to provide amine group sites on the cellulosic fibers. Polydimethylsiloxane-isocyanate (PDMS-NCO) was applied as a water-repellent outer layer, which was bonded to cellulosic fibers via biodegradable urea linkages formed between the NCO and amine groups. The obtained coated paper substrate showed excellent water-resistant properties with high water contact angles ( 0303125°) along with a significant reduction in water absorption. However, the coating paper was lacking oil resistance, which is evidently due to the porous textures of the coated paper.To impart paper substrates both water- and oil-resistant properties, a two-step PFAS-free approach was developed using chitosan as a first layer, while PDMS-NCO as a second layer Chitosan was applied as a coating to mask paper's pores as well as create free amine groups on the paper substrate. A PDMS coating layer was then chemically grafted to the chitosan-coated-paper via urea linkages to render water and oil-resistant. The coated paper with a coating load of 8.6% of chitosan and 2.2% of PDMS showed excellent oil/grease resistance (kit rating value of 12/12) as well as excellent water contact angle (WCA 95.2°). Solvent extraction tests confirmed the robustness of the coating on the cellulosic substrates.Next, a closed-loop approach for one-step water- and oil-resistant paper-based materials was developed. Chitosan-graft-PDMS copolymer was prepared by grafting PDMS onto the chitosan polymer, which was then applied as an aqueous coating solution on paper substrates in a single step. Remarkable water- and oil-resistant properties were obtained which is evident from high WCA of 120.53°, low Cobb 60 values of 9.89 g/m2 and very high kit value of 11.7/12. In addition, the pulp of the coated paper was 100% recyclable by repulping and washing treatment. Furthermore, overall migration of siloxane into two food simulants (50% aqueous ethanol and Miglyol 812) was below 46.19 [mu]g/mL (7.16 mg/dm2), which is well below the overall migration limit 60 mg/kg food (100 mg/dm2) by the Council of Europe's Resolution AP (2004). The migration studies indicate the potential safe use of the coated paper for food contact application.Finally, sustainable and biobased chitosan-graft-castor oil (CHI-g-CO) copolymer was synthesized and applied for paper coating. A castor oil-capped-isocyanate (CO-capped-NCO) was prepared by the reaction between castor oil and isophorone diisocyanate. The resultant CO-capped-NCO was grafted onto the chitosan polymer backbone via a urea linkage formed between -NCO and amine groups to prepare CHI-g-CO. The obtained waterborne solution of CHI-g-CO was coated onto paper, which showed good water- and oil-resistance with a Cobb60 value of 29.16 g/m2 and a kit rating value of 9/12, respectively. Meanwhile, this fabrication strategy offers a sustainable and environmental-friendly approach toward grease- and water-resistant paper-based materials considering the biobased nature of chitosan and castor oil.